The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory captions and links exemplifying features within the Earth system. The community is invited to contribute digital imagery, short captions and relevant links.

Lichens and Standing Stones

How long do lichens live? That question crossed my mind when visiting the Standing Stones of Stenness (above) on the Scottish island of Orkney. I wondered if the lichen that we see there today could be the same lichens that lived on these rocks when the henge was built. When we returned home I did a little research on the stones and lichen.

Lichens grow slowly: Estimates of annual growth rates vary from 0.02 in (0.5 mm) to 20 in (500 mm). Lichen reproductive techniques are so various that it's impossible for a nonspecialist to determine the extent of any individual lichen, much less its age. Nevertheless, let’s throw caution to the wind and do some math. The penny in the photo at left is about 20 mm in diameter. It could take 25 – 40 years for a lichen to grow to the size of the penny. The patch of greenish lichen below the penny is representative in terms of area and thickness of the lichen patches on the stones. It’s about 80 mm long and 20 mm wide and covers an area of about four pennies. At the suggested growth rates that would take between 100 and 200 years. But the stones have been standing there for over 5,000 years! By any stretch of the imagination there does not appear to be enough lichen on the stones for all that time.

The answer may be in an old photo taken in 1930 (bottom left) and found in the British Geological Survey’s Archive of Geological Photographs. It shows the stones much more heavily encrusted with lichen than they are today. The answer to my question then is that the lichens of today are most likely much younger than the date the stones were erected. It would appear that sometime between 1930 and 2011 someone decided to clean up the Stones of Stenness so they'd look nice and tidy for the tourists, like the well-kept green lawn that now surrounds them. In 2011 over 140,000 tourists visited Orkney. The Standing Stones of Stenness site is currently administered by Historic Scotland and is part of a UNESCO World Heritage site. Photos taken September 24, 2011 and January 6, 1930.